1. Field of the Invention
This invention relates to transducers for use in transmitting or receiving ultrasound that is propagated through sheet material in on-line applications. More particularly, the invention relates to such transducers that employ a piezoelectric element having the shape of a parallelepiped and being separated from the sheet material by a protective nosepiece that is ridigly attached to the element and adapted for contact with the sheet material.
2. Description of Prior Art
On-line applications for transducers have typically employed cylindrical piezoelectric elements, although elements having numerous other shapes, including that of a parallelpiped, are known. It is also known that in on-line applications it is desirable to separate the element from the sheet material in order to preserve the useful properties of the element.
U.S. Pat. No. 3,792,294 Royer describes a rectangular AT-cut quartz resonator designed for the frequency range of 1 to 6 MHz and having a prescribed length-to-thickness ratio, width-to-thickness ratio, and orientation in order to eliminate the effects of combination of vibration modes.
U.S. Pat. No. 4,509,153 Weight described several ultrasonic transducers designed to transmit or receive "edge waves" without substantial transmission or reception of "plane waves." Although the invention describes circular sources of propagation, it suggests that the active surfaces of the source may follow two parallel lines (Col. 9, 1. 21-23).
U.S. Pat. No. 4,562,372 Nakamura, et al. describes a piezoelectric vibrating element of rectangular configuration for which width-to-length ratios are selected to correspond to the negative peaks of capacitance ratios in an equivalent circuit.
U.S. Pat. No. 4,291,577 Baum, et al. describes a transducer used in an on-line application that transmits ultrasound into sheet material and receives ultrasound therefrom. The transducer uses two piezoelectric elements with an electrode between the two, and an aluminum button through which ultrasound is transmitted to and from the sheet. The piezoelectric elements are described as sandwiched between the aluminum button and an aluminum ballast, and the shape of the elements is not specifed. As practiced, the invention employs cylindrical elements.
In on-line applications transducers typically use cylindrical piezoelectric elements that produce either a radial motion or a piston-like motion that is perpendicular to the sheet and that produces flexural waves in flexible sheet material. In both cases, compressional and shear waves are produced as primary or secondary vibrational effects. In any radial direction, a receiving transducer therefore detects both modes of propagation in the plane of the sheet, making it difficult to derive a measurement based on a particular mode.
The transducers of the instant invention produce resonant vibration such that, when two receiving transducers are positioned relative to a transmitting transducer as hereinafter described and all are in simultaneous contact with the sheet material, each receiving transducer detects ultrasonic waves of predominantly one mode of propagation.